FOOD90023 · Food Microbiology
Microbial Diversity
Bacteria are not the only microbes in food. This chapter covers the other two groups the subject cares about. Fungi (yeasts and moulds) are eukaryotes with a chitin wall that feed by absorption — they both spoil food (mycotoxins, gas, surface films) and make it (bread, beer, wine, cheese); you need to classify a fungus by wall chemistry and reproduction, contrast yeast budding with mould hyphae and spores, and know how a fungal (reproductive) spore differs from a bacterial endospore. Bacteriophages are viruses that infect bacteria, and the lytic vs lysogenic fork — burst the host now, or hide quietly as a prophage — decides whether a dairy starter culture survives. The phage long-answer (definition → cycles → starter-culture failure → biocontrol / therapy → advantages and disadvantages) is a guaranteed exam question.
What this chapter covers
- 01Fungal biology — chitin wall, absorptive nutrition, ergosterol membrane
- 02Yeasts vs moulds — budding vs hyphae/mycelium and spores
- 03Mycotoxins and the spoilage vs fermentation roles of fungi
- 04Fungal (reproductive) spores vs bacterial (survival) endospores
- 05Bacteriophage structure and the lytic cycle order
- 06Lytic vs lysogenic — the prophage and starter-culture failure
- 07Phages in food — biocontrol, biosanitisation, therapy; advantages and disadvantages
Worked example: bacterial vs fungal spores — the headline comparison
- +1(a) Purpose. A bacterial endospore is for survival — it lets one cell wait out heat, drying and chemicals. A fungal spore is for reproduction and dispersal — it spreads the organism to new substrates.
- +1(b) Number per cell. A bacterium makes one endospore per cell (so the count does not increase). A mould makes many spores from one organism (so the count multiplies).
- +1(c) Resistance. Bacterial endospores are extremely heat- and chemical-resistant (they survive boiling). Fungal spores are far less resistant — closer to a vegetative cell — and are usually killed by normal cooking.
- +1(d) Consequence — bacterial. Because endospores survive cooking, spore-formers like Clostridium set the canning standard (the 12-D botulinum cook).
- +1(d) Consequence — fungal. Because mould spores spread the organism and some moulds make mycotoxins, mould growth is mainly a spoilage and toxin problem controlled by drying, low aₜ and chilling, not by a heat process.
Key terms
- Bacteriophage (phage)
- A virus that infects and replicates inside bacteria. It hijacks the host's machinery; its genome can take the lytic road (replicate then burst the cell) or the lysogenic road (integrate as a quiet prophage). Phages cause dairy starter-culture failures and are used as biocontrol agents.
- Lytic vs lysogenic cycle
- The two phage strategies. Lytic: adsorption → injection → biosynthesis → maturation → lysis (the host bursts, releasing new phages). Lysogenic: the phage genome integrates into the host chromosome as a prophage and replicates quietly with the cell until it switches to the lytic cycle.
- Prophage
- A phage genome integrated into the host bacterium's chromosome during the lysogenic cycle. It replicates along with the host DNA and is passed to daughter cells, lying dormant until a trigger switches it to the lytic cycle.
- Hyphae / mycelium
- The thread-like filaments (hyphae) that a mould grows as; a mat of them is a mycelium. This filamentous growth distinguishes moulds from yeasts, which are single cells that reproduce by budding.
- Mycotoxin
- A toxic secondary metabolite produced by some moulds (e.g. aflatoxin from Aspergillus) that can contaminate food. Mycotoxins are a key reason mould growth is a safety problem, not just a quality one, and many are heat-stable so they persist after the mould is killed.
Microbial Diversity FAQ
Why is the bacteriophage question 'guaranteed'?
Because the coordinator recycles it: phages appear as a long-answer in both the mid-semester and the final example papers. A full-mark answer follows a fixed arc — define a phage (a virus that infects bacteria), describe the lytic and lysogenic cycles (with the prophage), explain a food consequence (dairy starter-culture failure when a phage lyses the lactic acid bacteria), then give the beneficial uses (biocontrol of pathogens, biosanitisation of surfaces, phage therapy) with their advantages (highly specific, self-replicating) and disadvantages (narrow host range, possible resistance). Pre-write that paragraph and you bank a big block of marks.
How do I avoid confusing fungal spores with bacterial endospores?
Anchor on purpose: fungal spores are for reproduction and dispersal (many per organism, easily killed by cooking), while a bacterial endospore is for survival (one per cell, extremely heat-resistant, survives cooking). They share the word 'spore' but are opposite in number, resistance and food-safety meaning.
Why are fungi able to spoil dry or acidic foods that bacteria cannot?
Moulds tolerate lower water activity and lower pH than most bacteria, so they grow on dried, salted, sugared and acidic foods (jams, bread, hard cheese rinds) where bacterial growth is hurdled out. That, plus their absorptive feeding and mycotoxin production, makes them the dominant spoilers in those products.
What makes a phage useful as a biocontrol agent in food?
Its specificity. A phage typically infects only one species or strain, so it can be applied to kill a target pathogen (e.g. Listeria on ready-to-eat surfaces) without disturbing the rest of the microbial community, and it self-replicates while the host is present. The trade-offs are that narrow host range (you need the right phage for the target) and the possibility of bacterial resistance.
Exam move
Treat this chapter as two guaranteed answers plus one comparison. First, pre-write the phage long-answer in the fixed order (define → lytic/lysogenic with the prophage → starter-culture failure → biocontrol / biosanitisation / therapy → advantages and disadvantages, with a named lactic acid bacterium) — it is the single highest-yield paragraph in the subject. Second, be able to classify a fungus by its chitin wall, absorptive nutrition and reproduction (budding yeast vs hyphal mould), and recite the lytic cycle order. Third, drill the bacterial vs fungal spore comparison until it is automatic (survival vs reproduction, one vs many, very resistant vs not), because it appears here and again in the foundations and inactivation chapters.
